1. Field of the Invention
The present invention relates to a printer focusing method capable of being used with a printer which is arranged to determine a printing size by altering the position and conjugate length of a printing lens.
2. Description of the Related Art
Photographic films are of several types: 135 film (35 mm format), 120 film (Brownie film), 110 film and so forth. In addition, the same photographic film includes a full-frame format and a half-frame format. To handle these various formats, a known type of printer is arranged to alter a printing size by moving a printing lens and by altering the conjugate length of a printing optical path. The amount of movement of the lens and the conjugate length can be obtained from an AF curve. The following is a description of a conventional method for obtaining the AF curve.
FIG. 3 is a schematic view of a printer. A negative film 60 is irradiated by a light source (not shown). The light is transmitted through the negative film 60 and irradiates a photographic paper 62 through a lens 62, thereby effecting printing on the photographic paper 64. In this case, the following equation is obtained from a first lens formula ##EQU1## where A is distance between the lens and the negative film,
S is conjugate length, and PA1 f is focal length of the lens. PA1 A' is distance between the lens reference and a lens position, PA1 Sp is distance between the negative film and a photographic-paper reference (compensation value), PA1 S' is distance between the photographic-paper reference and a photographic-paper position, PA1 A'm1 and S'm1 are A' and S' at the magnification m1, and PA1 A'm2 and S'm2 are A' and S' at the magnification m2.
Next, a magnification m1 is specified and, if the lens is focused at the magnification m1, the following equation is obtained from the above equation (1) ##EQU2##
If a magnification m2 is specified and the lens is focused at the magnification m2, the following equation is likewise obtained ##EQU3## where Ap is distance between the negative film and a lens reference (compensation value),
Incidentally, A' and S' are values which are obtained by actual distance measurement (for example, by counting the number of feed pulses of a pulse motor). Ap and Sp are values which are obtained by calculations performed when it is assumed that the above equation (1) is obtained.
In this case, since A'm1, S'm1, A'm2 and S'm2 are the distances from the respective reference positions, their values are known, and the focal length f of each lens is also determined. Accordingly, Ap and Sp can be obtained from the above equations (2) and (3).
As can be seen from the following equation employing the thus-obtained Ap and Sp, if either Ap or Sp is determined, the other value can easily be obtained. ##EQU4##
In typical printers, it is generally required that focus and magnifications be accurately controlled. Accordingly, in an actual printing operation, it is necessary that accurate focus is obtained when a magnification has been input.
More specifically, if the amount of defocusing is within the permissible circle of confusion, the lens is regarded as focused. However, since the magnification varies within the in-focus range of the lens, compensation corresponding to the variation is needed.
Therefore, A' and S' are calculated at a predetermined magnification in accordance with the following procedures.
First, the following equation is obtained as a second lens equation ##EQU5## Then, as in the case of focusing, Am and Sm are obtained from the following equations ##EQU6##
By obtaining Am and Sm from equations (6) and (7), if either Am or Sm is determined, the other value can be obtained in the following equation ##EQU7##
A' and S' are obtained from the simultaneous equations of equations (8) and (4).
The optimum values of Ap, Sp, Am and Sm are obtained by repeating the aforesaid focusing operations, and these optimum values are stored in a memory in a control section. Subsequently, only when a magnification m is input is it possible to easily obtain A' and S' which enable focusing at the magnification m.
However, such a focusing method has the following problems. If the lens is to be moved near an in-focus position, an operator operates a fine adjustment screw attached to a lens holder for focusing purposes. Accordingly, the amount of fine adjustment varies depending upon the manner of operation of each operator. It is difficult to return the lens position to a selected optimum focus position. As a result, operability is deteriorated. In addition, it is necessary to visually check the state of focus while projecting an image. However, such visual checking is difficult in the case of printers of the so-called normal vertical arrangement type in which a negative film is disposed above the photographic paper. On the other hand, the range of application of such a method is limited to printers of the so-called inverted vertical arrangement type in which a negative film is disposed below the photographic paper. Furthermore, where an image projected from a negative film is to be visually checked, no exact judgment as to focus can be performed since the luminous intensity of the image is low.
For these reasons, the present applicant proposes a method of obtaining a correct AF curve by altering a lens position within a predetermined range, effecting printing at each lens position, selecting a lens position at which optimum focus is obtained, and compensating for an AF curve on the basis of this lens position and a real magnification. These steps are repeated until the optimum focus position reaches the mid position of the aforesaid processing range (Japanese Patent Application No. 172381/1987).
In this proposed method, the AF curve is substantially automatically compensated for, the operation of an operator is simplified, and any error due to the manual operation of the operator is prevented.
However, this automatic focusing method has the following problems. Since focusing is performed together with the setting of a magnification, a relatively long focusing time is required to obtain an optimum focus position at a predetermined magnification. Therefore, the efficiency of operation is inferior. In addition, when a magnification corresponding to the optimum focus position is to be input, an operator may input an inaccurate value. Accordingly, there may be cases where no correct AF curve can be obtained.